Field of the Invention and Prior Art Statement
The present invention relates to a bivalent signal generating circuit for converting an input signal having a substantially sinusoidal waveform into a bivalent signal having high and low levels. Particularly, the bivalent signal generating circuit according to the present invention is suitable for converting into a bivalent signal, a tracking error signal, which is obtained from an optical pick up device for reading out information recorded on an information recording medium such as an optical disc and an optical card.
Hitherto, in an information recording and/or reading out apparatus using the information recording medium such as the optical disc and the optical card, the optical pick up device is moved in a tracking direction, i.e. a direction across tracks formed on the information recording medium, in order to search information recorded thereon at a high speed. In this case, the tracking error signal is used for counting the number of tracks, across which the light beam has transversed, and for detecting a moving velocity of the light beam on the recording medium. That is to say, the tracking error signal is converted to the bivalent signal by comparing the tracking error signal with a given threshold level in a comparator, and then the numbers of the thus converted bivalent signals is counted to measure the numbers of the tracks over which the light spot has moved and the period of this bivalent signal is detected to measure the moving velocity.
However, the tracking error signal does not always have a correct sinusoidal waveform. If a condition of a surface of the optical information recording medium is not good or there is an interference between recording pits, a noise is sometimes superimposed upon the tracking error signal. Thus, when the tracking error signal, on which the noise has been superimposed, is converted to the bivalent signal with the aid of the comparator having the single threshold level, a chattering is caused in the vicinity of the given threshold level due to the noise. Accordingly, counting of the number of tracks and detecting of the moving velocity of the light beam cannot be carried out correctly.
In order to remove the above mentioned chattering there has been proposed a comparator having a hysteresis property.
FIG. 1 is a block diagram showing such a bivalent signal generating circuit for use in converting the tracking error signal to the bivalent signal. As shown in FIG. 1, in the conventional bivalent signal generating circuit, a comparator circuit with hysteresis 50 is used as the comparator for producing the bivalent signal. FIG. 2 is a circuit diagram depicting the construction of one example of the comparator circuit with hysteresis 50. As depicted in FIG. 2, in the comparator circuit with hysteresis 50, the tracking error signal S.sub.1 is supplied to a non-inverted input terminal of a comparator 51 via a resistor 53, while an inverted input terminal of the comparator 51 is connected to the ground. An output signal of the comparator 51 is connected to the non-inverted input thereof via a resistor 52 to form a positive feedback loop; and the hysteresis function is realized thereby.
However, in the conventional bivalent signal generating circuit 50, a hysteresis width is usually set such that when a signal level of the tracking error signal is substantially constant, it is possible to effect a proper conversion. However, in practice, the signal level, i.e. a peak-to-peak value of the tracking error signal is varied over a wide range, so that the conversion could not be performed in a proper manner. That is to say, since the noise level has a tendency to vary in proportion with the signal level of the tracking error signal level, it is necessary to determine the hysteresis width large so as not to be influenced by the noise even when the signal level of the tracking error signal becomes maximum. While, in order to produce the bivalent signal even when the signal level of the tracking error signal becomes minimum, it is required to make the hysteresis width small. That is to say, in the known comparator, the threshold levels could not be varied in accordance with the peak-to-peak value of the tracking error signal. As shown in FIG. 3A, under the condition that the large hysteresis width is given as shown by broken lines, when the tracking error signal S.sub.1 having a large signal level is supplied to the comparator circuit 50, the bivalent signal S.sub.2 can be obtained in a proper manner because the tracking error signal S.sub.1 varies beyond the threshold levels; but when the tracking error signal S.sub.1 having its signal level smaller than the hysteresis width, as illustrated in FIG. 3C, the output signal S.sub.2 of the comparator 51 is not changed and the bivalent signal could not be obtained correctly.
Particularly, in a magneto-optic disc, there are provided prepit regions in which address information of tracks of the disc is preliminarily recorded, so that a large noise is superimposed upon the tracking error signal at the prepit regions. Therefore, where a magneto-optis information recording medium is used, the problem mentioned above is apt to be caused.